The present invention relates to a purifier of a noxious gas containing all kinds of hazardous components or malodorous components.
There are noxious gases containing hazardous components or malodorous components such as ethylene oxide gas used for sterilization procedure in hospitals or the like, carbon monoxide or hydrocarbon etc. exhausted from e.g. combustion equipment or from drying equipment for organic solvents. These hazardous components and these malodorous components should be purged from noxious gases before the noxious gases are released to the atmosphere. One approach to purify a noxious gas is to pass the noxious gas through an oxidation catalyst. The use of an oxidation catalyst can accelerate the oxidation reaction of ethylene oxide gas, carbon monoxide, hydrocarbon or the like at a lower temperature (for example, 200 to 400xc2x0 C.) than that for burning directly, so that the noxious gas can be changed into innocuous carbon dioxide or water vapor.
Such a gas purifier has, depending on e.g. the amount of catalyst stored inside, limitations with respect to the quantity of noxious gas components to be purified per unit of time (hereinafter referred to as xe2x80x9cprocessing capabilityxe2x80x9d). If a noxious gas in excess of the upper limit of this processing capability (concentration or gas volume) is supplied to the purifier, the noxious gas is exhausted without being sufficiently purified. In using this type of gas purifier, it is therefore important to prevent the supply quantity of noxious gas components, even temporarily, from exceeding the processing capability of the purifier.
However, a noxious gas is not always produced in a constant quantity and in a constant concentration. It is rather common that the quantity and the concentration of gas produced fluctuate to a large extent. Moreover, depending on the release source, a noxious gas is often produced intermittently. In such a case where the quantity and the concentration of noxious gas fluctuate, the installation of a purifier with processing capability covering the peak quantity of noxious gas is not an economical solution from the viewpoint of cost of the device and space to be occupied by the device.
Thus, consideration has been given to equipping a purifier with a cushioning device (buffer), which supplies the noxious gas to the purifier after the changes of quantity and concentration of a noxious gas have been smoothed, namely reduced, as much as possible. The simplest example of such a buffer is the use of a variable-volume container such as a plastic bag. In this case, the produced noxious gas is gathered temporarily in a plastic bag and then fed in an almost constant quantity into a purifier by the operation of a suction fan or the like of the purifier.
However, even if the use of a variable-volume container as a buffer can reduce fluctuations in the quantity of the supplied noxious gas, fluctuations of noxious gas concentration cannot be controlled. In other words, it is not possible to prevent the concentration of noxious components (components to be purified) contained in a noxious gas from fluctuating. Furthermore, although a plastic bag is inexpensive, this plastic bag needs to have a considerably large size for demonstrating sufficient buffer effects. As a result, a large space is required for installation, and the installation method is also not easy. Additionally, for safety reason, the plastic bag may have to be placed inside a casing having a certain degree of mechanical strength, which forms a dual structure resulting in higher cost. There are also inflammable gases and explosive gases included in noxious gases, so that this aspect should be taken into account as well.
Accordingly, it is an object of the present invention to provide a purifier of a noxious gas equipped with a buffer means for purifying the noxious gas economically and safely when the quantity or the concentration of the noxious gas fluctuates.
In order to accomplish the above object, a purifier of a noxious gas of the present invention is provided with means for supplying a gas containing a noxious gas component, buffer means using a liquid which dissolves the noxious gas component in the supplied gas temporarily and later releases the dissolved noxious gas component, and means for purifying the noxious gas component in the gas supplied from the buffer means.
According to the purifier using such a liquid, the noxious gas component can be supplied to the purification means after having been reduced at least to a certain value. Furthermore, there is no need to install a special apparatus or facility for carrying out this process. The size of the device itself can be determined appropriately according to the required processing capability and will not be unnecessarily large. Furthermore, if a noncombustible and inactive liquid is chosen as the liquid to be used for the buffer means, even inflammable and explosive noxious gases can be treated safely.
Apart from the present patent application, the applicant of the present invention has filed another patent application of a purifier equipped with buffer means using a gas adsorbent such as activated carbon (Japanese Patent Application (Tokugan Hei) 9-250245). According to this device, the noxious gas can be supplied to the purification means after the concentration is controlled at least to a certain value. And the same with the purifier of the present invention, the device in the aforementioned other application also demonstrates excellent cost performance. However, a certain kind of noxious gas such as ethylene oxide generates heat (ca. 70 to 80xc2x0 C.) when adsorbed to activated carbon etc., so that the noxious gas may not be purified successfully. On the other hand, the purifier of the present invention has the advantage of solving this problem of heat generation by using a liquid for the buffer means.
In the present invention, a noxious gas implies not only a gas which is hazardous for organisms including human beings or harmful to the environment, but also to all kinds of gases that preferably are removed, such as a hazardous gas of inflammable or explosive nature, an unpleasant gas with offensive odor, and a gas containing this kind of hazardous and unpleasant gas. The gas may be inorganic or organic. As examples of noxious gases, there are a volatile organic compound gas such as above-mentioned ethylene oxide, toluene, and acetone etc., and an inorganic compound gas etc. such as ammonia or the like. Gases containing a noxious gas can be listed, for example, emission gas of a sterilization apparatus used for sterilizing medical instruments (ethylene oxide-containing gas), emission gas of an incinerator, emission gas of a plant, exhaust gas of an organic solvent dryer, exhaust gas of an incinerator, exhaust gas of a draft chamber, and exhaust gas of a paint booth.
In the purifier of the present invention, it is preferable that the liquid in the buffer means dissolves the noxious gas component when the concentration of the noxious gas component in the supplied gas is high and releases the noxious gas component dissolved thus far when the concentration is low. Accordingly, the noxious gas component can be supplied to the purification means in a constant concentration, and the purification can be carried out efficiently. Such a function originally should be given to the liquid itself. However, if this function is not carried out sufficiently, as will be described later, a gas containing a noxious gas or a clean gas may be blown into the liquid to force dissolution or release.
It is preferable that the purifier of the present invention includes means for supplying a clean gas, with which the clean gas is supplied into the liquid in the buffer means. By supplying the clean gas into the liquid in the buffer means, the noxious gas dissolved in the liquid can be released even more. As a result, this liquid is purified and can be used once again for dissolving a noxious gas with high concentration.
There are two ways of supplying a clean gas into the liquid in the buffer means. The first embodiment is to supply a gas containing a noxious gas and a clean air by turns, and the second embodiment is to supply a gas containing a noxious gas and a clean gas at the same time. Depending on the configuration of the device and so forth, it is possible to choose one or the other embodiment appropriately, but the second embodiment is preferable. This is because when a gas containing a noxious gas and a clean gas are supplied simultaneously into the liquid in the buffer means, the dissolution and the release of the noxious gas occur simultaneously. Thus, the liquid can demonstrate the buffer effect and control the concentration of the noxious gas to be supplied into the purification means at least to a certain value. Moreover, the processing time can be reduced considerably from that required for the above first embodiment.
In the purifier of the present invention, it is preferable that the buffer means includes means for bringing a gas released from a liquid once again into contact with the liquid. This configuration enhances the buffer effect, since the noxious gas contained in the released gas dissolves once again in the liquid.
In the purifier of the present invention provided with the means for conducting the repetitive contact mentioned above, in the case where the process of contacting once again and the process of supplying a clean gas into the liquid both are carried out in the same container, the clean gas should not be supplied into the liquid when the gas released from the liquid is brought once again into contact with the liquid. On the other hand, it is preferable to supply the clean gas into the liquid at the time when the gas released from the liquid is not brought once again into contact with the liquid.
In the purifier of the present invention provided with the means for conducting the repetitive contact mentioned above, it is preferable that the buffer means includes a dissolution section for dissolving a noxious gas into the liquid, a purification section for cleaning the liquid containing the dissolved noxious gas with a clean gas, and means for bringing the gas released from the liquid in the dissolution section into contact with the liquid cleaned in the purification section Accordingly, the gas containing the noxious gas can be supplied simultaneously with the clean gas, so that the processing time and the processing efficiency are improved significantly. In the buffer means of this purifier, it is preferable that the liquid circulates between the dissolution section and the purification section, and that the gas containing the noxious gas is fed into the purification means from at least the purification section of the two aforementioned sections. Consequently, the processing time and the processing efficiency are improved even more.
When the gas released from the liquid and the liquid are brought into contact once again, the purifier of the present invention preferably is equipped with means for increasing at least either the contact area or the contact time. This is because the re-dissolution process of the noxious gas with the liquid can be carried out even more efficiently.
For increasing the above contact area or the above contact time, it is preferable to use, for example, means to shoot the liquid in the form of fog or shower, or means to let the liquid run along the surface of a member. This member should have a large surface area, for example, a horseshoe-shaped (U-shaped) member made of ceramics, metals, or resin etc. By placing a large number of such members and by pouring the liquid over these members, the liquid runs along the surfaces of these members, thereby contacting and dissolving the noxious gas. This kind of member is generally available on the market as a member used for increasing a gas-liquid contact area. Besides the aforementioned horseshoe shape, there are other forms such as Raschig ring, partition ring, primtriangle or the like. Not only these members but also a member with a large surface area such as a porous member, a member made of fiber, or the like can be used. Preferably, the members used here should have a small pressure loss, that is, good permeability.
It is preferable in the purifier of the present invention that the purification section mentioned above cleans the noxious gas component by means of a catalyst. In comparing with the method for purifying a noxious gas by burning, the use of catalyst has not only the advantage of reduction in energy costs but also of superiority in safety. As for the catalyst mentioned above, for example, oxidation catalysts can be used. To name some concrete examples, precious metal catalysts such as platinum and palladium, magnesium oxide, and copper oxide can be used. Furthermore, these catalysts usually are supported by a carrier, for example, by calcium aluminates. A carrier generally is formed e.g. in a honeycomb construction with permeability, and a catalyst is supported by the carrier through impregnation and adhesion. The present invention prefers to use a platinum catalyst supported by calcium aluminates of honeycomb construction. Moreover, the amount of catalyst used etc. is not particularly limited and can be decided appropriately depending on the intended processing capability and other conditions.
It is preferable that the purification section mentioned above includes means for detecting a catalytic temperature and means for controlling the quantity of a noxious gas to be supplied into the purification means based on the temperature detected by the detection means of catalytic temperature. If the concentration of a noxious gas increases, the catalytic temperature may rise due to the accelerated catalytic reaction, and the aforementioned means are provided to prevent this from occurring.
In the present invention, the liquid used for the above buffer means preferably has low volatility, less harmfulness, and small heat release when burned. Examples of the above liquid are: aqueous solvents such as water and sodium hydroxide aqueous solution; organic solvents such as alcohols, e.g. methyl alcohol, ethyl alcohol, and polyalcohol derivatives etc., e.g. ethylene glycol. The above liquid may be used individually or in combinations of two or more different kinds. When two or more different kinds of liquid are used, they may be used in a mixed form or separately. These liquids are selected appropriately according to the kind of noxious gas and so forth. For instance, water is preferably used for ethylene oxide, methyl ethyl ketone, isopropyl alcohol or the like. While alcohols are used preferably for xylene, toluene, methyl isobutyl ketone or the like, ethylene glycol is used preferably for acetone.
In the present invention, if a noxious gas to be purified is ethylene oxide, water should be used as the above liquid for the reason described above. Additionally, if a noxious gas to be purified is ethylene oxide, the concentration of ethylene oxide contained in a gas supplied from buffer means to purification means is preferably controlled to be 20000 ppm or less. The lowest critical concentration of explosion is 3% for ethylene oxide, so that the concentration of 20000 ppm or less does not cause a safety problem. For other noxious gases, the lowest concentration of supply to the purification section is determined appropriately by the nature of the gas etc. For example, the concentration for methyl ethyl ketone is 1.8%, 2.0% for isopropyl alcohol, 1.0% for xylene, 1.2% for toluene, 1.4% for methyl isobutyl ketone, and 2.15% for acetone.
In the present invention, the above clean gas should be an inert gas, and for the reasons such as application and cost, it is preferable to use clean air. In the present invention, a clean gas implies a gas that substantially does not contain a noxious gas to be purified, and the use of a filter etc. for collecting dust is not particularly necessary. However, it is preferable to use a gas that has passed through a dust collecting filter for durability of the device or with respect to catalytic activity and so forth.
In the present invention, the means for supplying a gas into the liquid is preferably operated by blowing the gas into the liquid and producing a large quantity of bubbles. It is because this method improves the solubility of the gas into the liquid. One example of this means for producing bubbles is blowing the gas into the liquid through a porous member.
In the present invention, when a water-sealed vacuum pump is used for supplying a gas containing a noxious gas together with water, it is preferable that the water circulates between the water-sealed vacuum pump and the buffer means. This system can utilize the water of the water-sealed vacuum pump effectively.
This purifier preferably is configured such that, when an abnormal state breaks out, the water and the gas containing a noxious gas supplied from the water-sealed vacuum pump are not supplied into the buffer means but instead released to the outside of the purifier. Furthermore, it is preferable to store the released water and the released gas containing a noxious gas temporarily in a storage tank. After the purifier has returned to a normal state, the water and the gas to be purified are supplied from the storage tank into the buffer means and then cleaned by the purification means.
Next, the present invention provides a method for purifying a noxious gas by which a gas containing a noxious gas component is purified. The method includes the steps of: passing the gas through buffer means using a liquid that dissolves the noxious gas component temporarily and releases the dissolved component later, and supplying the above gas into purification means. Here, the gas supply is intermittent, or the concentration of the noxious gas component becomes intermittently higher. With this method, the gas is supplied to the purification means after the peak concentration value of the noxious gas component in the gas is reduced. This method can be carried out with the use of the purifier of the present invention described above. Due to the reasons mentioned above, it is preferable in this method to supply the gas containing the noxious gas and the clean gas simultaneously into the liquid in the buffer means.